In light source devices for liquid crystal projectors and DLP projectors which are required to be reduced in size and provide bright projection images, short arc type high pressure mercury vapor discharge lamps which are small in size and can provide light emission at high brightness have been used as the light source. Since the lamps of this type involve a problem that starting performance under cold conditions and restarting performance under hot restrike conditions are not generally preferred, a UV enhancer as a start assisting light source is provided to enhance the starting performance (refer to the Patent Document 1).
A high pressure discharge lamp 51 shown in FIG. 8 has an arc tube 53 and a UV enhancer 54 disposed inside an outer tube 52.
The arc tube 53 has a discharge chamber 56 formed at the central portion thereof in which a pair of tungsten electrodes 55 and 55 are opposed to each other and mercury, halogen such as bromine and a starting gas such as an argon gas are filled therein. A pair of electrode sealed portions 60A and 60B each having the electrode 55, a metal foil 58, and an electrode lead 59 sealed therein are formed from the discharge chamber 56 to both ends of the arc tube 53, and the electrode leads 59 and 59 protruding from the ends of the electrode sealed portions 60A and 60B are connected by way of power feed wires 61A and 61B to a base 62.
The UV enhancer 54 comprises a glow discharge tube 63 that emits UV-light to the discharge chamber 56 for promoting start up of lighting.
The glow discharge tube 63 has a seal portion 67 formed on one end of an airtight chamber that constitutes a light emitting chamber 64 for inserting and sealing an electrode rod 66 that constitutes an internal electrode 65A, and the electrode rod 66 protruding to the outside is welded to the power feed wire 61A on one side.
Further, a ring shaped external electrode 65B fixed by means of cement to the outer peripheral surface of the light emitting chamber 64 is welded by way of a connection wire 68 to the power feed wire 61B on the other side.
According to the constitution, since a starting high frequency pulse voltage supplied by way of the power feed wires 61A and 61B upon lighting of the arc tube 53 is applied between the internal electrode 65A and the external electrode 65B of the UV enhancer 54, glow discharge is caused in the discharge lamp 63 to generate the UV-light which is irradiated to the discharge chamber 54 of the discharge lamp 51 to improve the starting performance.
However, since the UV enhancer 54 of this kind is generally very thin as having a diameter of about 3 mm, when the electrode rod 66 is sealed in the seal portion 67, no sufficient thickness can be ensured the periphery of the electrode rod 66. Then, as shown in FIG. 9, when an external force is exerted in the direction of bending the electrode rod 66 protruding from the seal portion 67, since this is bent only at a corner 71 formed between an inner surface 69 of a through hole for the electrode rod 66 and an end face 70 of the seal portion 67 as a support point, this issued a problem that the bending stress is concentrated to the corner 71, for example, at a bending angle θ of the electrode rod 66 tending to cause cracks therein.
For preventing occurrence of cracks upon bending of the lead of such as the electrode rod protruding from the seal portion of the discharge tube, it has been proposed to form a concave hole 72 to the end face of the seal portion 67 and form a protrusion end 73 of the seal portion 67 into a sleeve-like shape (refer to the Patent document 2).
According to the constitution, as shown in FIG. 10, when an external force is exerted in the direction of bending the electrode rod 66 protruding from the seal portion 67, since the lead is bent at two positions, that is, at a corner 74 formed between the inner surface 69 of the through hole of the electrode rod 66 and a bottom of the concave hole 72 and at a corner 75 at the inner surface of the opening end of the concave hole 72 as support points, for example, the entire bending angle θ of the electrode rod 66 is divided into a bending angle θ1 at the corner 74 and a bending angle θ2 at the corner 75 and, accordingly, the bending stress is also dispersed for the respective corners 74 and 75, cracks are less caused.
However, since formation of the concave hole required many steps, it was troublesome and time consuming to result in a problem that not only the manufacturing cost was increased but also the yield was poor.
Previously, as shown in FIG. 11(a), a non-fusing agent 77 was coated over the electrode rod 66 so as to have an outer diameter equal with the inner diameter of the concave hole 72, the electrode rod 66 is inserted from one end of a quartz tube 78 and the seal portion 67 is heat-sealed by using a burner 79 as shown in FIG. 11(b), the quartz tube 78 was cut to a predetermined length by using a grinding wheel 80 or a file and polishing the non-fusing agent 77 coated over the electrode rod 66 as shown in FIG. 11(c).
Then, a glazing treatment of head-melting again the tubular end of the quartz tube 78 by using the burner 79 as shown in FIG. 11(d), and amending fine scars or cracks formed by the grinding wheel 80 or the like is performed and then a chipping-off treatment of chipping-off the upper end of the quartz tube 78 to form the light emitting chamber 64 was performed.
As described above not only the non-fusing agent 77 has to be coated over the electrode rod 66 but also the coated the non-fusing agent 77 has to be removed by polishing after cutting off the quartz tube 78, as well as twice heat treatment is necessary, including a heat treatment of softening the quartz tube 78 for sealing the electrode rod 66 and a glazing treatment of heating the tubular end after cutting the quartz tube 78 are necessary.
Further, since the UV-light power of the UV enhancer depends on an electric field formed between the external electrode and the internal electrode, the UV-light can be outputted more efficiently as the area of the external electrode is larger when the potential difference between both of the electrodes is equal.
However, in a case of using the ring-shaped external electrode 65B as shown in FIG. 8, when the width of the ring is made large for increasing the UV-light output, since the light emitting chamber 64 of the glow discharge tube 63 is covered by the external electrode 65B, this gives a problem that the UV-light is shielded and the UV-light cannot be irradiated by a necessary and sufficient amount to the discharge chamber 56 of the lamp 51.
Accordingly, the present applicant trially manufactured the UV enhancer capable of efficiently irradiating a UV-light by a necessary and sufficient amount into the discharge chamber of the high pressure discharge lamp without shielding the UV-light even when the area of the external electrode is increased (Japanese Application No. 2009-130211).
FIGS. 12(a) and (b) show such a UV enhancer 81. A flat pinch seal portion 84 is formed on one end of a glow discharge tube 83 in which an airtight chamber that constitutes a light emitting chamber 82 is formed on the other end, and an electrode assembly 85 comprising an internal electrode 85b and an electrode lead 85c welded on both sides of a sealing metal foil 85a is sealed in the pinch seal portion 84.
An external electrode 86 disposed to the outside of the light emitting chamber 82 comprises a holder 87 formed by bending fabrication of a metal plate so as to grip the light emitting chamber 82, and a slit 88 for irradiating the UV-light to the discharge chamber and a clip 89 for holding the pinch seal portion 84 by gripping both the surface and the rearface thereof are formed to the holder 87.
According to the UV enhancer 81, as shown in FIG. 12(b), since the external electrode 86 that functions as the holder 87 covers the outer peripheral surface of the light emitting chamber 82 so as to grip the chamber, the area of the external electrode 86 can be ensured sufficiently and the UV-light can be irradiated efficiently.
Further, since the slit 87 is formed, when the slit 87 is disposed being directed to the discharge chamber of the high pressure discharge lamp, the UV-light is not shielded by the external electrode 86.
However, since the pinch seal portion 84 is formed flat, when the glow discharge tube 83 having the external electrode 86 attached thereto is mounted to the discharge lamp, if the external electrode 86 as the holder 87 is positionally displaced and the clip 89 approaches extremely to the lead 85c of the electrode assembly 85, insulation distance cannot be ensured between the electrode lead 85c and the external electrode 86 applied with a voltage at opposite polarity, which may possibly cause creeping discharge or atmospheric discharge at the outside of the glow discharge tube 83.
Further, since the electrode assembly 85 is sealed by a flat pinch seal portion 84, no sufficient strength can be ensured in the direction of the thickness and cracks tend to be caused in the pinch seal portion 84 when the electrode lead 85c is bent in the direction of the thickness. In addition, when it is intended to form a concave hole in the end face of the pinch seal portion 84 by the method described in the Patent Document 2, there occurs another problem that such concave hole cannot be formed since there is no sufficient margin in the direction of the thickness of the pinch seal portion 84.